Monday, June 16, 2008

Ocean changes may trigger US megadrought

From NewScientist Environment:

CALIFORNIANS will hope the drought in their state won't last as long as the parched period that afflicted North America from AD 800 to 1250. Even if it passes, global warming may yet hasten another 500-year "megadrought".

Song Feng and colleagues at the University of Nebraska at Lincoln built a climate model to simulate the way changes in sea surface temperature of a few degrees in the Atlantic or Pacific can disrupt atmospheric circulation over North America. Such shifts caused droughts such as the 1930s Dust Bowl, which followed a cooling of the tropical Pacific.

The team found that the impact of these sea surface temperature changes differs by season. The effects of a change in the Pacific would hit mainly in winter: ocean cooling of 3 °C would reduce the occurrence of winter storms. Meanwhile, effects of changes in the Atlantic would strike mainly in summer: warming of 1 °C would reduce the transport of moisture to the Great Plains of the central US and western parts of the continent. When both these effects occur together, North America suffers a megadrought, says Feng...MORE

We've been posting on these long cycle changes since it was confirmed by NASA that the PDO had flipped back to the cool phase:

...There are several theories as to what caused the demise of the Moche political structure. Some scholars have emphasised the role of environmental change. Studies of ice cores drilled from glaciers in the Andes reveal climatic events between 536 to 594 AD, possibly a super El Niño, that resulted in 30 years of intense rain and flooding followed by 30 years of drought, part of the aftermath of the climate changes of 535–536.[3]These weather events could have disrupted the Moche way of life and shattered their faith in their religion, which had promised stable weather through sacrifices.

The BBC had a show on the Moche a while back. Here's what they say happened:

...If the weather on the coast was the opposite, then it suggested a 30-year El Nino - what climatologists call a mega El Nino – starting at around 560 AD, which was followed by a mega drought lasting another 30 years. Such a huge series of climatic extremes would have been enough to kill off an civilization – even a modern one. Here, at last, was a plausible theory for the disappearance of the Moche. But could it be proved?...

It turns out that the Moche adapted to the 30 years (that's a PDO cycle folks) of floods and the 30 years of drought which followed.They ended up killing themselves after surviving all that:

...Dillehay now put together a new theory. The Moche had struggled through the climaticdisasters but had been fatally weakened. The leadership - which at least in part claimed authority on the basis of being able to determine the weather – had lost its authority and control over its people. Moche villages and and/or clan groups turned on each other in a battle for scare resources like food and land. The Moche replaced ritual battles and human sacrifices with civil war. Gradually they fought themselves into the grave....